1. Field of the Invention
The present invention relates to a crystal growth method and, more particularly, to a sapphire crystal growth method.
2. Description of the Related Art
A conventional sapphire crystal growth method comprises a flame fusion process, wherein the aluminum oxide powder is falling down and melted by a torch which is formed by burning hydrogen and oxygen. The liquid aluminum oxide is dripped onto a pan and then solidified to form a single-crystal aluminum oxide having a diameter reaching about 3 centimeters. However, such a single-crystal aluminum oxide contains air bubbles and an aluminum oxide powder that has not been melted, and has a residual stress, thereby decreasing the quality of the sapphire crystal.
Another conventional sapphire crystal growth method comprises a flux growth process, wherein the aluminum oxide is melted at a temperature under 2050° C. by aid of an assistant, such as the plumbum (Pb) oxide, aluminum fluoride and sodium fluoride. Then, the liquid aluminum oxide is cooled and saturated. However, the flux growth process is not available for mass production in the industry.
Another conventional sapphire crystal growth method comprises a Czochralski process, wherein the aluminum oxide powder is placed in a crucible, and the crucible is heater by a graphite resistance so that the aluminum oxide powder is melted in the inert gas or in the vacuum. Then, the sapphire crystal falls down gradually to touch the liquid surface slightly. At this time, the sapphire crystal is rotated slowly and drawn upwardly to draw out the sapphire crystal.
Another conventional sapphire crystal growth method comprises a thermal gradient technique which is researched by the Crystal System company in 1978. The thermal gradient technique includes a crucible in a vacuum resistance furnace, a thermal body and a shielding device. However, the thermal gradient technique produces a molybdenum (Mo) pollution during the crystal growth.
Another conventional sapphire crystal growth method comprises an edge-defined film-fed growth (EFG) process, wherein the sapphire crystal is not rotated during the crystal growth, and the diameter and shape of the sapphire crystal is controlled by a die floating at the liquid surface of the aluminum oxide. However, the sapphire crystal contains a residual stress with a high density and incurs a dislocation, so that the edge-defined film-fed growth (EFG) process is not available for the semi-conductor substrate.
Another conventional sapphire crystal growth method comprises a Kyropoulos process, wherein the sapphire crystal is not rotated during the crystal growth and is not drawn upwardly. On the contrary, the sapphire crystal is solidified, cooled and shrunk in the crucible. However, the size of the sapphire crystal is slightly smaller than that of in the crucible, so that the sapphire crystal needs a complicated working procedure to reach the final diameter.
Thus, the sapphire crystal grown by the above-mentioned growth methods has limited mass and size, has a lower quality, has a higher price and has a longer growing time.